Among various techniques which are presently under test for implementing broadband telecommunications networks, asynchronous time division (ATD) techniques are of particular interest, since ATD systems are the only networks, which, at least from a theoretical point of view, can integrate more services at different speed by a single technique. Since this technique is based upon the switching of packets with a destination label, it is also referred to as "Fast Packet Switching" or "Label Addressed Switching" technique.
Highly promising networks allowing the use of label addressed switching techniques, are described in the paper entitled "ATD Switching Networks" from Proceedings of GSLB-Seminar on Broadband Switching - Albufeira, Portugal, Jan. 19-20, 1987, pages 225-234.
Such networks are based on small-sized switching elements (typically 2.times.2), organized so that the packet is self-routing through the network; each stage need examine only one bit of the label, deciding on the basis of its examined value to which of the two outputs the packet is to be forwarded.
Since a network of this type is typically blocking, each network node needs a buffer memory, where the packet can wait when its destination output has been seized by the other input. As a consequence, the network is not time-transparent and its efficacy is higher if the entering traffic is randomly distributed and the ratio between presence and absence times of the signal is low. By current technologies (C-MOS) and 8 channels in parallel the global throughput of a 128.times.128 network can be a few Gbit/s.
Throughput can be increased by other technologies: an increase of an order of magnitude can for instance be obtained by ECL. An alternative can be the optical technology. However, from the switching point of view, the latter technology offers the systemist rather limited performance: matrices with a small number of inputs and outputs (8.times.8 is considered significant), but with rather large size (a few cm); high input/output attenuation (some dB) and high crosstalk (few tens of dB).
The most promising devices for the optical switching commercially available nowadays are based on directional couplers or on X-junctions, generally obtained by diffusing titanium optical guides in a substrate of lithium niobate.
Devices with X junctions are described in the paper entitled "Survey of Optical Switching", at pages 143-151 of the previously-cited Proceedings and directional couplers are described in the paper entitled "High Speed Optical Time-division and Space-division Switching" issued in the Proceedings of IOOC-ECOC 85, Venice, Oct. 1-4, 1985, pages 81-88.
Nowadays, devices of this type can allow limited-capacity matrices to be implemented (e.g. 12.times.12 with directional couplers, 16.times.16 with X-junctions). The X-junction seems to be better adapted to matrix organization, since it has no bending losses, but needs higher driving voltage.
The reduced performances of these elements are compensated for by a very large bandwidth (some tens of GHz) and a quasi-infinitesimal switching time (some tens of ps). These characteristics make interesting the use of optical elements in switching networks wherein having a normally small size and wherein the bandwidth can be used to increase the ratio between presence and absence times of the signal.
So far neither optical logic devices (to be used for selfrouting functions in the network) nor optical memory elements of practical utility are available. Hence with this background it would not seem that in the near future optical switching elements would be used as in electrical technology.